1Put the sugar and water in a saucepan. Stir over medium heat until the sugar has dissolved. Increase the heat to medium high and bring the syrup to a simmer. Simmer for 2 minutes. Transfer to a large heatproof measuring cup. Allow to cool.
2Stir the grapefruit juice into the syrup. Cover and refrigerate 3 hours until cold.
3Pour the chilled mixture into the ice cream bowl. Set the ice cream maker to the desired setting and churn.
4Once the sorbet is frozen, spoon or scoop into chilled bowls or glasses and serve or transfer to a freezer-safe container, just large enough to contain the sorbet. Freeze for 2-3 hours or overnight for firmer sorbet.
5Store sorbet in the freezer for up to 3 weeks. The sorbet will become very firm after freezing overnight, so remove from the freezer and stand at room temperature for 5-10 minutes before serving for easier scooping.